Hoo boy, this is getting weird. People treating me like an expert an' stuff, me and
siren gearing up for a slug-out of the
Lords of Space-time, and now requests for me to review nodes about physics...
oy...
I didn't scream, but that's cos it's 2 am and my sister's ill, and plus the WU's not that bad, iainb.
General relativity suggests that the best explanation for certain inertial phenomena is a 4-dimensional space-time which is curved by the presence of mass. The more mass in an area, the more curved space-time in that area is.
Simply put, a singularity is an infinitesimal point of infinite space-time curvature. It doesn't have infinite mass (if it did it would eat the rest of the universe - the whole curvature of space-time thing would go berko everywhere to an infinite degree) but it does have infinite density, because zero volume. It's not considered part of 'our' space-time beacause all the laws of physics that apply in our space-time break down at a singularity - you may as well think of it a being a gateway to the end of the universe.